Here is a thread I made 5 years ago that might help you.

On top of that stuff, most important thing is to remember to call the game! Don't worry too much about what's your area, just call the game as you would normally. As you gain more experience you will understand your coverage area better and know where to look when. Definitely get video of yourself doing 3 person and analyze your game. Become your worst critic and ask for advice and fix the stuff you don't like.

If you're scared about missing a rotation as Trail when Lead goes over just remember if the ball is on the far side of the court (even above the FT line extended), then as Trail you need to look off ball. And most times when you look off ball you will see the Lead rotating across and will be able to pick it up!